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Captain Video · Joined: 09 Jun 2003 Posts: 37 Location: Ft. Worth, TX

Ok, the noise is getting louder. The car is a 1977 924. I don't want to just start replacing parts without really knowing if they are bad. The symptoms are above 30mph, the car makes a high pitched whumping noise that is primarily present only when you just let off the accelerator, but before the car begins to engine brake. Under acceleration there is no noise. The noise is a constant frequency of whumping that seems to be at a higher rate than what I would expect with tire rotation noise. Could it be CVC joints? I don't get any clunking noise when I shift gears. Wheel bearings? The noise does not increase or is not present with side lateral movement like I would expect with a bad bearing. What does drive shaft bearing noise sound like when it is going out? And last but not least I don't know if it could be the transmission? Wouldn't the transmission do this at all speeds????? I have crawled under the car and tried to shake the axles to see if there is any play - none. I have pulled the brake drums and looked in there, but nothing appears to be loose or requiring repair. Can you please help. Thanks

Paul · Posted: Tue Feb 06, 2007 1:27 am Post subject:

Have you checked your tranny's oil level?

Does the thumping increase with engine speed (tach) or wheel speed (speedo)?

Does it occur in all gears or just high gear?
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

Captain Video · Joined: 09 Jun 2003 Posts: 37 Location: Ft. Worth, TX

Paul,
The tranny's oil level is normal (no leaks), now that you mention it the whumping noise increases the faster I go - still not sure if this is wheel or bearing noise. I'm primarily in high gear when it is really loud, but again it doesn't begin until above 30mph at which point I'm in the lower gears. Any ideas?

Smoothie · Posted: Tue Feb 06, 2007 3:38 am Post subject:

"Whumping" would be a good description of the sound I heard from another car on the highway many years ago.. This cars' rear-left wheel was actually bouncing, apparently from being way out of balance. -So I'd suspect a wheel balance problem.
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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

Captain Video · Joined: 09 Jun 2003 Posts: 37 Location: Ft. Worth, TX

Smoothie,
I've experienced what you describe when I was much younger, an out of balance tire or a bubble on the tire will make a "whumping", but it will also affect the ride of the car - usually with a vibration and shake of the automobile. This "whumping" sound does not affect the ride of the car it is like a background din that keeps getting louder (from when it started a few months ago). Any ideas anyone?????? I would prefer not to spend tons of money on new axles, cvc joings or wheel bearings only to find that it was the last item on the list and not the first two (hope this makes sense).

!tom · Posted: Tue Feb 06, 2007 7:32 am Post subject:

Getting your tires ballanced sounds like a good first step; it's fairly inexpensive to do, and if a rear tire is out part way, it may be much less noticable vibration wise than a front tire, since the fronts affect the steering.

From there, progress along the drivetrain in a sensible manner.

Could there be an issue with the brakes? I'm having a hard time correlating a "whump" sound with any noise I could imagine coming from a car.

If it changes in frequency depending on engine speed, you know it's before the transmission, and if it changes in frequency depending on road speed, you know it's after. The most important thing to do is determine which side of the transmission you need to look at. Does the sound go away when you put the clutch in and coast on the highway? Is there a difference when coasting fast on the highway in gear with the clutch in or in neutural with the engine at idle? This will allow you to isolate the torque tube.
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78 924 NA
5-lug

Ozzie · Posted: Tue Feb 06, 2007 7:36 am Post subject:

Starting at the easiest, pull and repack wheel bearings and CV's with grease- probably long overdue anyway.
If its still there I'd suspect torque tube bearings.
Is the noise there in gear, neutral, clutch in or out? Accel or decel?
If still there in neutral with the clutch pedal depressed I'd suspect the wheel bearing or CVs.
_________________
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Captain Video · Joined: 09 Jun 2003 Posts: 37 Location: Ft. Worth, TX

Thanks Tom and Ozzie - I'll check the clutch in/out. The noise seems to be constant just increases in frequency as the speed of the car increases - again this is only when the car is in that area where you are not accelerating nor engine braking - you know when you see someone pulling out way ahead of you and you release part of the pressure on the gas pedal to see what they are going to do? - but you don't pull off so hard that the car engine brakes - it is in that spot that the noise comes on. Anyway, I will try your suggestions - Ozzie, the CVC joints were regreased about a year ago.

Thanks

D Hook · Joined: 02 Nov 2002 Posts: 3158 Location: Omaha, NE

Steel belt in one of the tires? Sometimes they'll break and cause a whumping sound, especially at highway speeds.

Just one more thing to check.....

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'80 924 n/a SOLD

Captain Video · Joined: 09 Jun 2003 Posts: 37 Location: Ft. Worth, TX

Tom and Ozzie, I checked out the idea of depressing the clutch with the tranny in neutral, the noise is still there. Does this mean it is in the torque tube or can I eliminate that area and concentrate on one of the other locations you mentioned to check? The noise gets louder the faster I go, but again only when the car is not accelerating or engine braking. Thanks

Lizard · Posted: Wed Feb 07, 2007 1:40 am Post subject:

can you tell if the noise is in the rear or the front?

I am guessing it sounds more like the rear, and a tire with an egg or seperated belt has the ability to not affect the ride when it is a rear tire, so swap the tires (both sides) front to rear and see if the noises location and or ride quality changes.
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